Reacting Gas-particle Flows Research Articles

A penalization method for the simulation of weakly compressible reacting gas-particle flows with general boundary conditions

In this paper, the Brinkman penalization method is extended to the weakly compressible formulation of the Navier–Stokes equations to study gas-particle flows with reactions and coupled heat and mass transfer. The weakly compressible approximation removes the acoustic modes from the solution while allowing a variable density. The Brinkman volume penalization method describes the solid phase as a porous medium with a vanishing permeability. The method is validated with literature benchmarks for a fixed or moving particle. Various reaction scenarios are then investigated. First, the capability of the method to deal with intra-particle diffusion and reaction is evaluated. The impact on the conversion of the particle Reynolds number based on the slip velocity is assessed, for a range of Reynolds numbers encountered in fluidization. Next, surface reactions are focused on, with infinite or finite rate. A Dirichlet-type condition is imposed on the particle surface to treat an infinite reaction rate and a high solid thermal conductivity while a finite-rate surface reaction requires Neumann and Robin surface conditions for the temperature and species mass fractions. The impact of density changes induced by heat release and molecular weight changes is assessed in these different cases. It is shown that the combination of the weakly compressible approximation and the penalization method allows to treat the presence of the solid phase and reactions in an efficient manner and to take into account the effects of compressibility in commonly encountered situations in reaction engineering.

Two-phase Turbulence Models in Eulerian-Eulerian Simulation of Gas-particle Flows and Coal Combustion

Eulerian-Eulerian (two-fluid) simulation of gas-particle flows and coal combustion are widely used because of its convenience in simulating large-size facilities. The key point is that it needs more complex closure models, compared to those in Eulerian gas-Lagrangian DEM modeling of particles. This keynote lecture will give a brief review on our many-year studies to solve these problems. The first one is the particle turbulence model. To overcome the shortcomings of the Hinze-Tchen's “particle-tracking-fluid” model, about 20 years ago, a transport equation of particle turbulent kinetic energy and transport equations of both gas and particle Reynolds stresses were proposed by us and subsequently constitute the so-called “k-ɛ-kp”, “unified second-order moment (USM)” and “non-linear k-ɛ-kp” two-phase turbulence models. Furthermore, for simulating reacting gas-particle flows and coal combustion, a full two-fluid model and a combined two-fluid-trajectory model, accounting for both particle turbulent diffusion and particle history effect due to moisture evaporation, devolatilization and char oxidation were proposed. The next is the particle-rough wall interaction. A particle-wall collision model accounting for wall roughness was proposed. Then, to overcome the limitation of the well-known kinetic theory of dense gas-particle flows, an anisotropic two-phase turbulence model, called “USM-Θ” model, accounting for both particle turbulence (large-scale fluctuation) and inter-particle collision (small-scale fluctuation) was proposed. Next, the particle-wake effect on gas turbulence modulation was studied to construct a sub-model and was added to the two-fluid modeling. At last, in recently developed two-fluid large-eddy simulation of gas-particle flows and combustion the particle sub-grid-scale (SGS) stress model is insufficiently studied. Some of them are based on a simple extension of the gas Smagorinsky SGS model without theoretical justification. Therefore, a USM-SGS two-phase stress model was proposed by us, properly accounting for the anisotropy of two-phase SGS stresses and the interaction between them.

Filtered models for reacting gas–particle flows

Using the kinetic-theory-based two-fluid models as a starting point, we develop filtered two-fluid models for a gas–particle flow in the presence of an isothermal, first-order, solid-catalyzed reaction of a gaseous species. As a consequence of the filtering procedure, terms describing the filtered reaction rate and filtered reactant dispersion need to be constituted in order to close the filtered species balance equation. In this work, a constitutive relation for filtered reaction rate is developed by performing fine-grid, two-fluid model simulations of an isothermal, solid-catalyzed, first-order reaction in a periodic domain. It is observed that the cluster-scale effectiveness factor, defined as the ratio between the reaction rate observed in a fine-grid simulation to that observed in a coarse-grid simulation, can be substantially smaller than unity, and it manifests an inverted bell shape dependence on filtered particle volume fraction in all simulation cases. Moreover, the magnitude of the deviation in the cluster-scale effectiveness factor from unity is a strong function of the meso-scale Thiele modulus and dimensionless filter size. Thus coarse-grid simulations of a reacting gas–particle flow will over-estimate the reaction rate if the cluster-scale effectiveness factor is not accounted for.

Studies of the effect of a coal concentrator on NO formation in swirling coal combustion

To develop low-pollution burners, the effect of a coal concentrator on NO formation in swirling coal combustion is studied using both numerical simulation and experiments. The isothermal gas–particle two-phase velocities and particle concentration in a cold model of swirl burners with and without coal concentrators were measured using the phase Doppler particle anemometer (PDPA). A full two-fluid model of reacting gas–particle flows and coal combustion with an algebraic unified second-order moment (AUSM) turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion and NO formation with different coal concentrators. The results give the turbulent kinetic energy, particle concentration, temperature and NO concentration in cases of with and without coal concentrators. The predicted results for cold two-phase flows are in good agreement with the PDPA measurement results, showing that the coal concentrator increases the turbulence and particle concentration in the recirculation zone. The combustion modeling results indicate that although the coal concentrator increases the turbulence and combustion temperature, but still can remarkably reduce the NO formation due to creating high coal concentration in the recirculation zone.

Simulation of coal combustion by AUSM turbulence-chemistry char combustion model and a full two-fluid model

An algebraic unified second-order moment (AUSM) turbulence-chemistry model of char combustion is introduced in this paper, to calculate the effect of particle temperature fluctuation on char combustion. The AUSM model is used to simulate gas-particle flows, in coal combustion in a pulverized coal combustor, together with a full two-fluid model for reacting gas-particle flows and coal combustion, including the sub-models as the k-ε- k p two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, and the six-flux radiation model. A new method for calculating particle mass flow rate is also used in this model to correct particle outflow rate and mass flow rate for inside sections, which can obey the principle of mass conservation for the particle phase and can also speed up the iterating convergence of the computation procedure effectively. The simulation results indicate that, the AUSM char combustion model is more preferable to the old char combustion model, since the later totally eliminate the influence of particle temperature fluctuation on char combustion rate.

04/00396 Simulation of swirling coal combustion using a full two-fluid model and an AUSM turbulence-chemistry model: Zhou, L. X. et al. Fuel, 2003, 82, (8), 1001–1007

Abstract A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion. The sub-models are the k – e – k p two-phase turbulence model, the EBU–Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The prediction results are in good agreement with the experimental results taken from references.

Simulation of swirling coal combustion using a full two-fluid model and an AUSM turbulence-chemistry model ☆

A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion. The sub-models are the k– ε– k p two-phase turbulence model, the EBU–Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The prediction results are in good agreement with the experimental results taken from references.

A multi-fluid model for simulating turbulent gas–particle flow and pulverized coal combustion

Based on the pure two-fluid model for turbulent reacting gas–particle flows with combusting pulverized coal particles, a new comprehensive model for pulverized coal combustion was developed by incorporating a modified k– ϵ– kp model, a general model of pulverized coal devolatilization and a general model of char combustion. Both gas-phase and particle-phase conservation equations are described using Eulerian coordinates, and these equations are discretized and integrated in the computational cell. As the first stage of numerical modeling of pulverized coal combustion in the cyclone furnace, three-dimensional simulation of turbulent gas combustion and gas–particle flows has been made. The predicted results show that there is a near wall recirculating zone at the bottom of the cyclone furnace, and the recirculating zone enhances ignition and flame stabilization. The predicted tangential velocity distribution of both the gas phase and the particle phase similar to those of the Rankine vortex.

Two-Fluid Models for Simulating Reacting Gas-Particle Flows, Coal Combustion and NOxFormation

Abstract Unlike the widely used Eulenan gas-Lagrangian particle models (particle trajectory models), two versions of two-fluid models—a pure two-fluid (FTF) model and a two-fluid-trajectory (continuum-trajectory, CT) model are proposed for simulating turbulent reacting gas-particle flows and coal combustion. Both of diem are based on Eulerian gas-phase equations, Eulenan particle-phase continuity and momentum equations, κ-ϵ-κ two-phase turbulence model, EBU-Arrhenius turbulent combustion model, six-flux radiation model, coal moisture evaporation, devolatilization and char combustion models with simultaneous three reactions. To account for the particle history effect, including the mass change due to moisture evaporation, devolatilization and char combustion and the particle temperature change due to the heat transfer between two phases, the FTF model uses Eulerian or partial differential conservation equations of particle mass, particle daf-coal mass, particle moisture and energy, while the CT model uses ...

96/01897 The character of ash deposits and the thermal performance of furnaces

Eulerian-Eulerian (two-fluid) simulation of gas-particle flows and coal combustion are widely used because of its convenience in simulating large-size facilities. The key point is that it needs more complex closure models, compared to those in Eulerian gas-Lagrangian DEM modeling of particles. This keynote lecture will give a brief review on our many-year studies to solve these problems. The first one is the particle turbulence model. To overcome the shortcomings of the Hinze-Tchen's “particle-tracking-fluid” model, about 20 years ago, a transport equation of particle turbulent kinetic energy and transport equations of both gas and particle Reynolds stresses were proposed by us and subsequently constitute the so-called “k-ɛ-kp”, “unified second-order moment (USM)” and “non-linear k-ɛ-kp” two-phase turbulence models. Furthermore, for simulating reacting gas-particle flows and coal combustion, a full two-fluid model and a combined two-fluid-trajectory model, accounting for both particle turbulent diffusion and particle history effect due to moisture evaporation, devolatilization and char oxidation were proposed. The next is the particle-rough wall interaction. A particle-wall collision model accounting for wall roughness was proposed. Then, to overcome the limitation of the well-known kinetic theory of dense gas-particle flows, an anisotropic two-phase turbulence model, called “USM-Θ” model, accounting for both particle turbulence (large-scale fluctuation) and inter-particle collision (small-scale fluctuation) was proposed. Next, the particle-wake effect on gas turbulence modulation was studied to construct a sub-model and was added to the two-fluid modeling. At last, in recently developed two-fluid large-eddy simulation of gas-particle flows and combustion the particle sub-grid-scale (SGS) stress model is insufficiently studied. Some of them are based on a simple extension of the gas Smagorinsky SGS model without theoretical justification. Therefore, a USM-SGS two-phase stress model was proposed by us, properly accounting for the anisotropy of two-phase SGS stresses and the interaction between them.